1. Field of the Invention
The present invention relates to an optical wavelength control method which makes possible architecture of a high density wavelength division multiplexing (WDM) system by controlling with high accuracy the optical wavelength of optical signals supplied by an optical transmitter used in the WDM system and thereby stabilizing the optical wavelength. The invention further enables optical communication systems to be reduced in size. The invention is particularly suitable for a wavelength control method by which, where a wavelength monitoring section for constantly monitoring the wavelength is integrated into a light source section, wavelength fluctuations due to degradation or any other factor are restrained.
2. Description of the Related Art
In optical fiber communication, wavelength multiplexing is used for economical transmission of a greater quantity of information. FIG. 1 illustrates a model of this method. An optical transmission system 1 on the sending side uses an optical transmitter 6 having different optical wavelengths xcex1 through xcexN and an optical multiplexer 2 to wavelength-multiplex individual optical signals onto a single fiber 3. An optical transmission system 5 on the receiving side uses an optical demultiplexer 4 to demultiplex the optical signals. In this manner, the capacity of an existing transmission line can be economically increased using existing optical fibers. In the drawing, reference numeral 7 denotes an optical receiver.
A further increase in transmission capacity can be achieved by broadening the band of wavelengths used. However, in order for optical fibers to transmit optical signals efficiently with little loss, the range of usable wavelengths is limited. Therefore, for multiplexing more optical signals, the wavelength spacing should be narrowed to achieve a higher density.
In today""s high density wavelength multiplexing systems, the oscillation wavelength of the semiconductor laser ranges between 1530 nm and 1560 nm. If adjoining wavelengths are spaced at 0.8 nm, about 40 waves can be wavelength-multiplexed. To achieve an even higher density, the wavelength spacing may be narrowed to 0.4 nm or even 0.2 nm to make possible wavelength multiplexing of 80 or 160 waves, respectively.
An important factor in increasing the density of wavelength multiplexing is the stability of wavelengths. If they are unstable, optical signals which are supposed to be independent from other optical signals of different wavelengths leak into signals of adjoining wavelengths, making it impossible to achieve a desired qualitative level of communication. Usually, the tolerable fluctuation in a wavelength multiplexing system of 0.8 nm in wavelength spacing is not more than 0.1 nm, and in a system of 0.4 nm or 0.2 nm the tolerance is about 0.05 nm or 0.025 nm, respectively.
The wavelength of a semiconductor laser used in a wavelength multiplexing system is heavily dependent on the temperature of the active layer of the semiconductor laser. Therefore, in order to enhance the stability of wavelengths, more accurate temperature control of the active layer of the semiconductor laser device is required.
Also, an optical transmitter used in optical communication is required not to fluctuate substantially in its average output intensity, and to realize this function there is used an auto-power control (APC) circuit. Even where the efficiency of light emission that can be expressed in the ratio between the output intensity of and the injected current to the semiconductor laser varies on account of the degradation of the laser or any other factor, this APC circuit controls the current injected to the laser so as to keep the average output intensity constant, resulting in a stable output intensity.
However, a variation in the current injected into the semiconductor laser device invites a variation in power consumption in the active layer of the semiconductor laser, resulting in a temperature change in the active layer. This temperature change in the active layer invites a variation in optical wavelength, posing a serious impediment to architecture of a high density wavelength multiplexing system.
Therefore, in order to realize a high density wavelength multiplexing system, a major requirement is to establish a temperature control method for keeping the temperature of this active layer constant.
FIG. 2 is a conceptual diagram illustrating an example of wavelength control method according to the prior art. The output optical signals of a light source 20 of an optical transmitter are branched into two segments using an optical splitter 11. Optical signals of one of these branched lights are further separated into two directions by another optical splitter 12. This semiconductor laser device is controlled in light intensity by an APC circuit. Optical signals in one of the directions into which separation was done by the optical splitter 12, after passing a wavelength filter 13 whose optical transmissivity is dependent on the optical wavelength, are received by a light receiving element (PD1) 14. Optical signals in the other direction are received directly by a light receiving element (PD2) 15 without going through the wavelength filter. A voltage Vpd1 generated by a photo current generated by the PD1 is similarly wavelength-dependent as shown in FIG. 3. Another voltage Vpd2 generated by the photo current generated by the PD2 is not wavelength-dependent because it does not go through the wavelength filter. FIG. 3 illustrates an example of the wavelength-dependence of the wavelength monitoring output, wherein the horizontal axis represents the wavelength, and the longitudinal axis represents the output Vpd.
By setting as desired the voltage Vpd2 to be generated by the PD2 and keeping the difference between Vpd1 and Vpd2 constant, the output optical wavelength of the laser can be set to a desired optical wavelength xcex1. Vpd1 and Vpd2 are received by a comparator 16, and the resultant error signal is fed back to a temperature control circuit 17 for controlling the laser temperature. The laser is mounted on a thermoelectric cooler (TEC) 19, and the temperature of the TEC 19 is controlled with a signal from the temperature control circuit 17. The temperature of the active layer of the semiconductor laser device is thereby kept constant. An example of optical wavelength stabilization system for keeping the optical wavelength of a laser constant by using a wavelength monitor circuit in this manner is disclosed in, for instance, a laid-open patent JP-A No. H11-251673.
According to the prior art, the section for controlling the temperature of the laser and that for monitoring its wavelength used to be configured as separate components. However, in response to a demand for smaller optical communication systems, integration of these laser mounting section and the wavelength monitoring section has been attempted. FIG. 4 illustrates how these components are packaged when they are integrated.
In an optical transmitter 40 illustrated in FIG. 4, a laser element 30, an optical splitter 31, a wavelength filter 32, a light receiving element 33 (PD3) and another light receiving element 34 (PD4) are mounted on a TEC 35 for controlling the temperature of the semiconductor laser element 30. The laser module with a wavelength monitor 39 is formed by comprising the laser element 30, the optical splitter 31, the wavelength filter 32, the light receiving element 33, 34 and the TEC. The light intensity of the semiconductor laser is controlled by an APC circuit 36. Control of the optical wavelength, as in the case of FIG. 2, is accomplished by comparing with a comparator 38 Vpd3 and Vpd4 generated by photo current flowing in the PD3 and the PD4, and feeding back the resultant error voltage to a temperature control circuit 37 of the laser. This serves to stabilize the wavelength. As the light source section and the wavelength monitoring section are integrated in this way, the optical transmitter can be reduced size, and so can be the wavelength multiplexing system using it.
Examples of component packaging where these light source section and wavelength monitoring section are integrated include what was described in a paper entitled xe2x80x9cA Turnable LD Module with a Built-in Wavelength Monitoring Sectionxe2x80x9d presented at the 2000 general convention of the Institute of Electronics, Information and Communication Engineers (in Japanese).
In the conventional laser module with wavelength monitor in which the light source section and the wavelength monitoring section are integrated, as illustrated in FIG. 4 cited above, the laser element 30 laser module with wavelength monitor, optical splitter 31, wavelength filter 32 and light receiving elements. 33 and 34 are mounted on the same TEC 35. If the efficiency of light emission that can be expressed in the ratio between the output intensity of and the injected current to the laser varies as a result of the degradation of the laser or any other factor, then the oscillation wavelength of the laser is controlled as described below.
The APC circuit 36 controls the injected current to the laser so as to keep the output intensity constant, and power consumption by the active layer of the laser varies, resulting in a change in the temperature of the active layer. As the temperature change of the active layer invites a change in the oscillation wavelength of the laser, the photo current generated by the PD3 through the wavelength filter varies. Since the efficiency of light emission by the laser usually drops as the laser performance deteriorates over time, the injected current to the laser increases, the temperature of the active layer rises, and the oscillation wavelength shifts toward the longer wavelength side.
The voltages Vpd3 and Vpd4 generated by the photo current of the PD3 and the PD4 are compared by the comparator 38, the resultant error signal is delivered to the temperature control circuit 37, and the laser element temperature is controlled so as to minimize the error. Accordingly, even after degradation, the oscillation frequency of the laser is controlled to stay at xcex1 all the time.
However, as a result of the integration of the wavelength monitoring section, the optical splitter, wavelength filter and light receiving elements are mounted on the TEC for controlling the laser temperature in addition to the laser, and the temperatures of these elements also vary when the temperature of the laser element is controlled with the TEC.
If the splitting rate of the optical splitter, the wavelength transmissivity of the wavelength filter and the light receiving efficiencies of the light receiving elements were not temperature-dependent, there would be no problem in wavelength control. However, all these characteristics are temperature-dependent. Particularly so is the wavelength transmissivity of the wavelength filter, and this factor accounts for a large part of the wavelength fluctuations of the laser module with a built-in wavelength monitor into which the wavelength monitoring section is integrated.
The process of wavelength fluctuations in the laser module with a built-in wavelength monitor into which the wavelength monitoring section is integrated is traced in FIG. 5. FIG. 5 illustrates the wavelength-dependence of the output of the laser module with wavelength monitor and variations in its characteristics with changes in ambience. Usually a semiconductor laser consisting of a material containing indium, gallium, arsenic or the like is used for optical communication, and the current required to be injected to achieve a desired light intensity is about 80 mA. When the efficiency of light emission by the semiconductor laser has fallen by 10% due to degradation, the APC circuit controls the injected current so as to keep the output intensity of the laser constant, resulting in a rise of the injected current to 88.9 mA. Then, as power consumption by the laser increases, the temperature of the active layer of the laser rises, usually by about 2xc2x0 C. under the above-described conditions. The control by the wavelength monitor circuit, comparator and temperature control circuit to keep the temperature of the active layer constant reduces the temperature on the TEC by 2xc2x0 C. from its initial level. The wavelength transmissivity of the wavelength filter is particularly temperature-dependent among the characteristics of the components mounted on the TEC. Where the wavelength filter is made of etalon, the temperature-dependence of its transmissible wavelength is about xe2x88x920.01 nm/xc2x0 C. Therefore, if the temperature of the TEC falls by 2xc2x0 C., the transmissible wavelength of the output voltage Vpd3 of the PD3 will shift by about xe2x88x920.02 nm toward the shorter wavelength side. However, as the output voltage Vpd4 of the PD4 remains unchanged, the optical wavelength will stabilize at xcex2, 0.02 nm away from the initial level xcex1 toward the shorter wavelength side.
As described above, keeping the wave stable with high accuracy is a vital requirement for a high density optical multiplexing system. In such systems, in which the wavelength spacing is 0.4 nm and 0.2 nm, the tolerable wavelength fluctuations are no more than 0.05 nm and 0.025 nm, respectively.
However, where a laser module into which the wavelength monitoring section is integrated is to be used, though the system size can be reduced, the temperature-dependence of the wavelength filter substantially expands the quantity of wavelength fluctuations, making it impossible to achieve a desired stability of wavelength stability.
In the conventional laser module with wavelength monitor into which the wavelength monitoring section is integrated, the laser chip, wavelength filter and light receiving elements are mounted on the same TEC. When the laser temperature is controlled the temperature control circuit, the temperatures of the optical splitter, wavelength filter and light receiving elements also varied. The wavelength transmissivity of the wavelength filter is especially temperature-dependent, and this poses a particular impediment to wavelength stabilization with high accuracy.
In order to solve the problem noted above, the aforementioned wavelength destabilizing factor can be eliminated by mounting the laser element on one TEC and the optical splitter, wavelength filter and light receiving elements on another independent TEC and subjecting the laser element-mounted TEC alone to temperature control by the temperature control circuit. However, temperature control of independent TECs would invite increased circuit dimensions and power consumption, which would run counter to the object of reducing the optical communication system size. Therefore, this option is excluded here as a means to solve the problem.
In the laser module with wavelength monitor into which the wavelength monitoring section is integrated, with reference to FIG. 4, the laser temperature is controlled so as to keep the difference between the voltages generated by the PD3 and the PD4 constant. Therefore, when the temperature of the wavelength filter varies and thereby invites a variation in the transmissible wavelength characteristic, the voltage Vpd3 generated by the PD3 varies as illustrated in FIG. 5, but the voltage Vpd4 generated by the PD4 does not vary, resulting in a change of the oscillation wavelength from xcex1 to xcex2.
In order to keep the optical wavelength unchanged, the voltage Vpd4 generated by the PD4, when the voltage Vpd3 generated by the PD3 has changed, may be controlled according to the quantity of the change in Vpd 3.
In the initial operation of the laser module with wavelength monitor, the state of its operation after degradation can be simulated by varying the current injected into the laser. The control voltage to restrain the wavelength variation then is stored in advance as compensation voltage into a memory, such as a semiconductor storage for instance, and the required compensation voltage is added to Vpd4 on the basis of this datum. If the injected current varies after degradation, a compensation voltage matching the variation is added to Vpd4, and the sum is compared with Vpd3 to provide a control signal to the temperature control circuit on that basis. In this way, even where the laser element, optical splitter, wavelength filter and light receiving elements are mounted on the same TEC, changes in characteristics due to temperature variations can be compensated for to give a stable optical wavelength xcex1. FIG. 5 illustrates this state.